There is known a circuit in which a signal input at a single end is amplified and converted to two signals having opposite phase polarities to each other. Such a circuit is herein referred to as a single-differential converting circuit.
FIG. 4 is a view illustrative of a conventional single-differential converting circuit. The single-differential converting circuit illustrated in FIG. 4 is provided with: a fully-differential type operational amplifier 104; two input impedance elements 101a and 101b respectively connected to an inverting input terminal 104a and a noninverting input terminal 104c of the operational amplifier 104; and two negative feedback impedance elements 102a and 102b respectively connected between the inverting input terminal 104a and a noninverting output terminal 104b, and between the noninverting input terminal 104c and an inverting output terminal 104d. Impedances of the input impedance elements 101a and 101b are both Z1, and impedances of the negative feedback impedance elements 102a and 102b are both Z2.
Additionally, Vip in the drawing indicates a voltage of a signal input to the single-differential converting circuit, and Von and Vop indicate voltages of signals output from the single-differential converting circuit. Vsp and Vsn are input voltages to the operational amplifier 104. “p” and “n”, each of which is a subscript indicative of physical quantity, are indicative of the phase of a voltage. The voltage indicated by the suffix “p” and the voltage indicated by the suffix “n” are ones having amplitudes inverted from each other with respect to a voltage value to be a direct current component of an alternating voltage as a reference. In other words, the phases are different by 180 degrees from each other.
Herein, the relationship between two signals having the phases different by 180 degrees from each other will be referred to as “reverse phase” or “opposite phase polarities”, and the relationship between two signals having the common phases will be referred to as “common phase” or “same phase polarity”. Additionally, the relationship between the common phase and the reverse phase will be referred to as “phase is inverted”. Furthermore, a first terminal assigned to an input and output of a signal and a second terminal assigned to a signal having an opposite polarity to the signal of the first terminal have a relationship of “opposite polarities”. Moreover, a first terminal assigned to an input and output of a signal and a second terminal assigned to a signal having the same polarity with the signal of the first terminal have a relationship of “same polarity”.
In the single-differential converting circuit illustrated in FIG. 4, the input voltage Vsp (≈Vsn) is varied in accordance with the change in the voltage Vip. For this reason, in the conventional art, it is necessary to design the single-differential converting circuit in consideration of the changes in Vsp and Vsn. This produces a problem in that there are limitations in the design condition of the single-differential converting circuit of, in particular, a low voltage.
As a conventional art for the purpose of solving the above problem, an example is Patent Document 1 that discloses a circuit illustrated in, for example, FIG. 5. In Patent Document 1, there is a relationship represented by the following mathematical expression among the voltage Vin of a signal input from the exterior and resistance values R1, R2, R3, and R4. Then, by setting the resistance values R1 to R4 to make smaller the common phase voltage VCM=(Vsp+Vsn)/2 in the expression, the common phase voltage VCM can be made small regardless of the voltage Vin.VCM/Vin=R3/[R1+R2(R1+R3)/(R2+R4)]·R2/(R2+R4)